Polymerization process

ABSTRACT

A process of preparing cation exchange resins by radiation grafting with copolymerization onto a perhalogenated fluorine-containing hydrocarbon polymer at least one functional monomer of formula 
     
         CF.sub.2 ═CF(CF.sub.2).sub.n A or 
    
     
         CF.sub.2 ═CF-O(CFX-CFX).sub.m A 
    
     and at least one non-functional monomer of formula CF 2  ═CFY, and optionally in the presence of a polymerization inhibitor and a chain transfer agent.

TECHNICAL FIELD

The present invention relates to novel cation exchange resins, theirpreparation and their use; in particular it relates to cation exchangematerials suitable for use as permselective membranes in electrolyticcells such as are used in the manufacture of alkali metal hydroxidesolutions and chlorine.

BACKGROUND ART

Alkali metal hydroxide solutions and chlorine are generally manufacturedin mercury cells or diaphragm cells. Mercury cells have the advantage ofproducing concentrated alkali metal hydroxide solutions but give rise toproblems associated with the disposal of mercury-containing effluents.On the other hand, diaphragm cells, in which the anodes and cathodes areseparated by porous diaphragms which permit the passage of both positiveand negative ions and of electrolyte, avoid the aforesaid effluentproblem, but have the disadvantage that:

(1) relatively weak impure alkali metal hydroxide solutions areproduced, which results in increased evaporation costs; and

(2) there is a possibility of product gases, namely hydrogen andchlorine, becoming mixed.

Attempts have been made to overcome disadvantages of both mercury cellsand diaphragm cells by the use of cells in which the anodes and cathodesare separated by cation-active permselective membranes; these aremembranes which are selectively permeable so as to allow the passage ofonly positively charged ions and not the passage of bulk electrolyte.Cation-active permselective membranes which are suitable for this use inchlorine cells include, for example, those made of synthetic organiccopolymeric material containing cation-exchange groups, for examplesulfonate, carboxylate and phosphonate.

In particular, synthetic fluoropolymers which will withstand cellconditions for long periods of time are useful, for example, theperfluorosulfonic acid membranes manufactured and sold by E I DuPont deNemours and Company under the trade mark "NAFION" and which are basedupon hydrolyzed copolymers of perfluorinated hydrocarbons (for examplepolytetrafluoroethylene) and fluorosulfonated perfluorovinyl ethers.

The active sites in the molecular structure of the resins from whichthese membranes are made are provided by the fluorosulfonatedperfluorovinyl ether component. These sites are present on side chainsattached by an ether linkage to the skeletal structure of the resin.Such membranes are described for example in U.S. Pat. Nos. 2,636,851;3,017,338; 3,496,077; 3,560,568; 2,967,807; 3,282,875 and UK Patent No.1,184,321.

Generally these fluoropolymers are made by the polymerization orcopolymerization of fluorocarbon monomers in an emulsion or suspensioncontaining a radical polymerization catalyst. The resulting polymers aremoulded into membranes by conventional moulding procedures such as meltfabrication.

DISCLOSURE OF INVENTION

We have now discovered a novel process of making fluoropolymers suitablefor use in cation-active permselective membranes. This process differsfrom the prior art processes in that monomers containing active sites,or functional groups that can be converted to active sites, are grafteddirectly onto perhalogenated polymeric skeletal substrates by a processof radiation grafting. It is a particular feature of our process thatthe polymeric skeletal substrates may be in powder or film form.

More particularly, we have found that when a functional monomer ashereinafter defined, a nonfunctional linking monomer as hereinafterdefined, and a perhalogenated fluorine-containing hydrocarbon polymerskeletal substrate are together subjected to a radiation graftingprocess the functional monomer and the nonfunctional linking monomersimultaneously copolymerize and graft to the polymer to form a cationexchange resin having a fluorine-containing hydrocarbon polymericsubstrate with pendant side chains containing functional groups. Thesefunctional groups may themselves be active sites for cation exchange ormay readily be converted to such active site by conventional processes,such as hydrolysis.

Accordingly we provide a process of preparing cation exchange resinswhich process comprises radiation grafting with copolymerization onto aperhalogenated fluorine-containing hydrocarbon polymeric skeletalsubstrate at least one functional monomer selected from the groupconsisting of compounds of formula I:

    CF.sub.2 ═CF(CF.sub.2).sub.n A

and formula II:

    CF.sub.2 ═CF-O-(CFX-CFX).sub.m A

wherein A is carboxyl, C₁ to C₆ -alkoxycarbonyl, hydroxy-C₁ to C₆-alkoxycarbonyl, cyano, hydroxysulfonyl, fluorosulfonyl, or the group--CO--NR¹ R² wherein R¹ and R² are independently selected from hydrogenand C₁ to C₆ alkyl, one X is fluorine and the other X is selected fromchlorine, fluorine and a trifluoromethyl group, n is an integer from 0to 12, m is an integer from 1 to 3; together with at least onenon-functional linking monomer selected from the group consisting ofcompounds of formula III:

    CF.sub.2 ═CFY

wherein Y is chlorine, fluorine, or a trifluoromethyl group.

The perhalogenated hydrocarbon polymeric skeletal substrate may beperfluorinated or partly fluorinated. A preferred fluorine-containinghydrocarbon substrate is a homopolymer or copolymer of a fluorinatedethylene, especially a homopolymer or copolymer of tetrafluoroethyleneor chlorotrifluoroethylene. Typical preferred substrates arepolytetrafluoroethylene (PTFE); polychlorotrifluoroethylene (PCTFE) andFEP which is the common name for the copolymer of tetrafluoroethyleneand hexafluoropropylene wherein the hexafluoropropylene incorporated inthe said copolymer is in the range of 3.5-12.5% w/w.

The preferred functional monomers of formula I for use in our processinclude pentafluorobutenoic acid, alkyl pentafluorobutenoates such asmethyl pentafluorobutenoate and ethyl pentafluorobutenoate, andtrifluorovinylsulfonyl fluoride. The preferred nonfunctional linkingmonomers of formula III are tetrafluoroethylene andchlorotrifluoroethylene.

The mixture of monomeric materials has to be in a liquid form and, ifnecessary, a common solvent is used to prepare a solution of them.Commonly one of the monomeric materials itself will provide the liquidphase dissolving the other monomeric material. Alternatively, withadvantage, the solvent used is one which will penetrate the substratematerial and cause it to swell, thereby allowing the solution ofmonomers to be absorbed right through the substrate material.

Suitable solvents are, for example, toluene and xylene, and chlorinatedhydrocarbons such as trichlorotrifluoroethane and oligomers oftetrafluoroethylene, for example, the tetramer and pentamer oftetrafluoroethylene. The substrate material may be pre-swelled with suchsolvents prior to the addition of the monomers.

Any of the known methods of radiation grafting may be employed. Forexample, the substrate and monomeric materials may be subjected togetherto continuous or intermittent radiation, or the substrate may bepre-irradiated prior to bringing it into contact with the monomericmaterials. Preferably the substrate and monomeric materials areirradiated together; the substrate, which is a solid and may be in theform of fine particles or as a sheet or film, is immersed in the liquidphase containing the mixed monomeric materials and the whole subjectedto irradiation by X-rays. electron beam, or preferably by γ-rays.

Preferably the grafting process is carried out in the absence of oxygen.

In those cases where a derivative of the active monomer is employed inthe grafting process, eg a carboxylic ester such as methylperfluorobutenoate, subsequent chemical treatment such as hydrolysis isrequired to convert the derivative into the active carboxylic acid form.

Ion-exchange resins prepared according to the present invention, haveenhanced properties particularly as regards resistance to degradation bywater uptake during use and find particular application in the form offilms as perm-selective membranes in electrolysis cells.

These membranes may be fabricated from particles of resin made by theprocess of the present invention. Preferably, a perhalogenatedfluorine-containing hydrocarbon polymeric film is first made and thenthis is subjected to the process of the present invention to form aresin of the present invention in the form of a membrane.

Accordingly, in a further embodiment of the present invention, there isprovided a perm-selective membrane, suitable for use in electrolysiscells, which comprises a resin having cation exchange properties,wherein the said resin is made by irradiation-induced copolymerizationof one or more functional monomers as hereinbefore defined, one or morenon-functional linking monomers as hereinbefore defined, to a substratecomprising a perhalogenated fluorine-containing hydrocarbon polymer,thereby forming a resin having a molecular structure consisting of aperhalogenated fluorine-containing hydrocarbon polymeric substrate withside chains comprising at least one active group derived from afunctional monomer.

The membranes according to this invention may also be usefully employedin other electrochemical systems, for example, as separators and/orsolid electrolytes in batteries, fuel cells and electrolysis cells.

The functional monomers, as hereinbefore defined, by themselves graftvery slowly and with low efficiency to the polymeric substrate. In ourprocess the addition of one or more non-functional monomers of formulaIII greatly increases the rate of grafting of the functional monomers.These non-functional monomers graft, under the polymerizationconditions, to both the polymeric substrate and the functional monomerand thus act as linking vinyl groups. Thus the grafted resins formed inour process contain side chains comprising groups formed from thefunctional monomers ("active groups") and groups formed from thenon-functional monomers ("linking vinyl groups").

The preferred ratio of active groups and linking vinyl groups in theside chains of the resins of our invention is in a molar ratio in therange of 2:1 to 1:20, and preferably in the range from 2:1 to 1:3. Toachieve these preferred ratios in the resins the range of functionalmonomers to non-functional monomers in the grafting process is in therange of 9:1 to 1:20, preferably in the range of 4:1 to 1:4, and morepreferably, in order to obtain the preferred resins of this invention,are mixed in nearer to equimolar proportions, that is in the range of2:1 to 1:2.

It is essential for the process of the invention that both thefunctional monomeric material and the non-functional monomeric materialare present together during the grafting process so that the freeradicals generated by the radiation may initiate both the grafting ofnon-functional groups to the substrate and, concurrently, thecopolymerization of the functional and non-functional monomericmaterials to form the chains which characterize the resins of thepresent invention.

BEST MODE OF CARRYING OUT THE INVENTION

The radiation grafting/copolymerization of the functional andnon-functional linking monomers onto the polymeric substrate appears tobe governed by two competing reactions. One of these is the desiredcopolymerization of the functional monomer with the non-functionallinking monomer which is simultaneously grafted onto the polymericsubstrate. The second is the copolymerization of monomers particularlythe non-functional linking monomers. Since the rate of copolymerizationmay be greater than the desired grafting/copolymerization, in apreferred embodiment of the process of radiation grafting withcopolymerization the functional and non-functional linking monomers ontofluorine-containing hydrocarbon polymeric substrate, ashereinbeforedefined, we provide the improvement comprising the additionof at least one polymerization inhibitor and at least one chain transferagent. In this embodiment higher levels of grafting can be achieved;typically the level of grafting is increased by a factor of three ormore. The resins from our improved process have higher ion exchangecapacity, and when such resins are incorporated into diaphragms for usein electrolytic cells, much better performance is achieved in thosecells.

The preferred polymerization inhibitors for use in the process of ourinvention include, for example, quinone inhibitors such asp-benzoquinone, naphthaquinone, and hydroquinone in the presence ofoxygen; inorganic inhibitors such as copper acetate; and compounds suchas 2,2,6,6-tetramethyl-4-oxo-piperidine-1-oxide,2,2,6,6-tetramethylpiperazine-N-oxide and chloranil.

The concentration of inhibitor used in the process of our invention isin the range of 0.001 to 2% w/w of the total mixture of functional andlinking monomers and charge transfer agents, preferably in the range of0.01 to 0.5% w/w.

Since the radiation grafting is preferably carried out in a liquidmedium it is preferable that the chain transfer agents are also solventsfor the monomers. Preferred chain transfer solvents include, forexample, chloroform, carbon tetrachloride, dimethylformamide andmixtures thereof. Suitable mixtures are for example, chloroform, carbontetrachloride, dimethylformamide and mixtures thereof. Suitable mixturesare for example carbon tetrachloride/chloroform (1:1) and carbontetrachloride/dimethylformamide (1:9). The concentration of monomers inthe chain transfer solvents is in the range of 10-60% w/w, preferably inthe range of 30-50% w/w.

Solid chain transfer agents are less preferred since additional solventsmay be necessary to provide a liquid medium for the radiation grafting.If solid chain transfer agents are used the w/w ratio of such transferagents to the monomers should be in the same range as that referred tohereinabove for the preferred chain transfer solvents.

It also lies within the scope of our invention to introduce furthercation exchange active groups to the resins, as hereinbefore definedcomprising a substrate, functional groups and non-functional groups. Theadditional active groups are introduced by chemical modification of thegroups already present. Thus, for example, the non-functional groups inthe side chains may be sulphonated and/or carboxylated to give activeresins having enhanced ion exchange capacity and wettability.

INDUSTRIAL APPLICABILITY

Hydrophilic diaphragms, according to the present invention, haveenhanced properties particularly as regards wettability by the liquidspresent in electrolytic cells and therefore they find particularapplication in electrolysis cells. They may also be usefully employed inother electrochemical systems, for example, as separators in batteries,fuel cells and electrolysis cells.

The invention is now illustrated by, but not limited to, the followingexamples in which all ion-exchange capacities are those relating tohighly alkaline conditions, ie all carboxylic acid and sulfonic acidgroups are acting as exchange sites. Unless otherwise stated all partsand percentages are on a weight basis.

The following general procedure was followed. A sample of perfluorinatedsubstrate was placed in a glass reaction vessel, and the monomer mixtureadded. The contents of the reaction vessel were frozen in liquidnitrogen and placed under vacuum to remove the air present in thesystem.

After thorough evacuation, the vacuum pump was disconnected and thecontents allowed to thaw and reach room temperature. This process,hereinafter referred to as degassing, was repeated three times beforesealing the reaction vessel.

Using this technique the contents of the reaction vessel were in avirtually oxygen-free atmosphere. Furthermore, the samples prepared,using this method, were then allowed to equilibrate at selectedtemperatures for a period of twenty-four hours.

After this time, the reaction vessel was transferred to an irradiationcell room and exposed to γ-rays emanating from a Cobalt-60 source.

After termination of the irradiation, the contents of the glass reactionvessel were frozen in liquid nitrogen prior to opening the reactionvessel. The grafted substrate (film or powder) was washed free ofunreacted monomer and homopolymer with a suitable solvent and dried in avacuum oven at 60° C. to constant weight.

In the Tables and description, the following abbreviations are used:

TFE=tetrafluoroethylene

FEP=copolymer of tetrafluoroethylene and hexafluoropropylene

PFBA=perfluorobutenoic acid

IEC=ion exchange

The sizes given are in microns and refer to thickness, in the case offilms, and particle size in the case of powders.

The percentage of graft (expressed as the weight increase of the film asa percentage of the weight of the grafted film), the infrared spectra ofthe grafted film (carbonyl absorption at frequency of 1795 cm⁻¹) and theion exchange capacity (expressed as meq/g) are used to characterize themodified perfluorinated substrate produced by γ-radiation.

The percentage grafting is calculated from the formula:

    % graft=[(G.sub.i -G.sub.o)/G.sub.i ]×100

where G_(o) is the initial weight of the polymer substrate and G_(i) isthe weight of grafted polymer after irradiation.

EXAMPLES 1-6

These examples illustrate the grafting of a mixture of functional andnon-functional monomers in the presence of a grafting inhibitorα-pinene.

FEP film (2.0 g) placed in a glass reaction vessel and PFBA, α-pinene(0.12 g, 0.5% total concentration), "Arklone" P, water (3.0 g) andammonium perfluorooctanoate (0.025 g, 0.17% total concentration) wereadded to the vessel. ("Arklone" is a Registered Trade Mark for1,1,2-trichloro-1,2,2-tri-fluoroethylene).

The mixture was frozen in liquid nitrogen, the air was evacuated andcontents allowed to come to room temperature. This process of degassingwas repeated three times, then TFE (free from any inhibitor) was chargedinto the reaction vessel at liquid nitrogen temperature. The totalweight of functional (PFBA) and non-functional (TFE) monomers andsolvent ("Arklone" P) was 21 g.

The glass reaction vessel was sealed and kept at room temperatureovernight. It was then irradiated at 10 KRAD/hr for 120 hours at ambienttemperature. The mixture received a total dose of 1.2 MRAD and it wasfrozen with liquid nitrogen again, prior to opening the glass vessel.The grafted film was collected, washed free of copolymers and unreactedmonomers and dried in vacuum oven at 60° C.

Various ratios of functional monomer, non-functional monomer andsolvent, were used.

The infra-red spectra of the grafted films showed the carbonylabsorption frequency to be 1795 cm⁻¹ and its ion exchange capacitydetermined by titration was 0.2 meq/g. The ratios used, the percentagegrafting, and the ion-exchange capacities are given in Table 1.

                  TABLE 1                                                         ______________________________________                                        Concentration (% w/w)*                                                                         Non-                                                                Functional                                                                              functional       %     IEC                                   Example                                                                              monomer   monomer   Solvent                                                                              Graft (meq/g)                               ______________________________________                                        1      37.5      12.5      50.0   49.6  0.22                                  2      25.0      25.0      50.0   58.6  0.18                                  3      12.5      37.5      50.0   78.0  0.08                                  4      50.0      16.7      33.3   44.6  0.25                                  5      60.0      20.0      20.0   40.7  0.29                                  6      75.0      25.0      --     52.1  0.34                                  ______________________________________                                         *the concentration in these tables is expressed as a percentage of the        total weight of functional and nonfunctional monomers and the solvent.   

EXAMPLES 7-13

The procedure of Example 4 was repeated except that the total absorbedradiation dose was varied as shown in Table 2 where the results aretabulated.

                  TABLE 2                                                         ______________________________________                                                 Absorbed     %        IEC                                            Example  Dose         Grafting (meq/g)                                        ______________________________________                                        7        1.20         40.7     0.30                                           8        1.75         47.5     0.36                                           9        3.50         51.8     0.41                                           10       10           68.5     0.41                                           11       30           68.5     0.32                                           12       75           72.7     0.24                                           13       180          88.6     0.12                                           ______________________________________                                    

EXAMPLES 14-20

The procedure of Example 6 was repeated except that the total absorbedradiation dose was varied as shown in Table 2 where the results aretabulated.

    ______________________________________                                                 Absorbed                                                                      Dose                  IEC                                            Example  (MRad)       % Graft  (meq/g)                                        ______________________________________                                        14       1.20         52.1     0.35                                           15       1.75         58.4     0.41                                           16       3.50         63.4     0.44                                           17       10           68.4     0.44                                           18       30           75.1     0.34                                           29       75           81.4     0.26                                           20       180          96.8     0.09                                           ______________________________________                                    

We claim:
 1. A process of preparing a cation exchange resin whichprocess comprises radiation grafting with copolymerization onto aperhalogenated fluorine-containing hydrocarbon polymeric skeletalsubstance at least one functional monomer selected from the groupconsisting of compounds of formula I:

    CF.sub.2 ═CF(CF.sub.2).sub.n A

and formula II

    CF.sub.2 ═CF-O-(CFX-CFX).sub.m A

wherein A is carboxyl or C₁ to C₆ alkoxycarbonyl, one X is fluorine andthe other X is selected from chlorine, fluorine and a trifluoromethylgroup, n is an integer from 0 to 12, m is an integer from 1 to 3;together with at least one non-functional linking monomer selected fromthe group consisting of compounds of formula III:

    CF.sub.2 ═CFY

wherein Y is chlorine, fluorine, or a trifluoromethyl group, the molarratio of functional monomer to non-functional linking monomer being inthe range of 9:1 to 1:20, said grafting being carried out with saidmixture of monomers in the presence of at least one polymerizationinhibitor and at least one chain transfer agent, such that least 40.7percent grafting is accomplished.
 2. A process according to claim 1wherein the fluorine-containing polymeric skeletal substrate is ahomopolymer or copolymer of a fluorinated ethylene.
 3. A processaccording to claim 2 wherein the fluorinated ethylene istetrafluoroethylene.
 4. A process according to claim 2 wherein thefluorinated ethylene is chlorotrifluoroethylene.
 5. A process accordingto claim 2 wherein the copolymer comprises hexafluoropropylene units. 6.A process according to claim 3 wherein the fluorine-containing polymericskeletal substrate is polytetrafluoroethylene.
 7. A process according toclaim 4 wherein the fluorine-containing polymeric substrate ispolychlorotrifluoroethylene.
 8. A process according to claim 2 whereinthe fluorine-containing polymeric substrate is a copolymer oftetrafluoroethylene and hexfluoropropylene wherein thehexafluoropropylene incorporated in the said copolymer is in theconcentration range of 3.5-12.5% w/w.
 9. A process according to anyclaim 1 wherein the compounds of formula I and II arepentafluorobutenoic acid and C₁ to C₆ alkyl pentafluorobutenoates.
 10. Aprocess according to claim 9 wherein the said C₁ to C₆ alkylpentafluorobutenoates are methyl pentafluorobutenoate and ethylpentafluorobutenoate.
 11. A process according to claim 1 wherein thenon-functional monomers of formula III are tetrafluoroethylene andchlorotrifluoroethylene.
 12. A process according to claim 1 wherein themolar ratio of functional monomer to non-functional monomer is in therange of 9:1 to 1:20.
 13. A process according to claim 12 wherein thesaid molar ratio is in the range of 4:1 to 1:4.
 14. A process accordingto claim 12 wherein the said molar ratio is in the range of 2:1 to 1:2.15. A process according to claim 1 wherein the radiation grafting is byirradiation by any one form of radiation selected from the groupconsisting of γ-rays, X-rays and electron beams.
 16. A process accordingto claim 1 wherein the mixture of monomers is dissolved in a solventcapable of swelling the substrate.
 17. A process according to claim 1wherein before addition of the monomers the diaphragm is treated with asolvent capable of swelling the diaphragm.
 18. A process according toclaim 16 wherein the solvent is selected from the group consisting oftoluene, xylene, trichlorotrifluoroethane and oligomers oftetrafluoroethylene.
 19. A process according to claim 1 wherein thepolymerization inhibitor is selected from the group consisting ofp-benzoquinone, naphthaquinone, hydroquinone in the presence of oxygen,copper acetate, 2,2,6,6-tetramethyl-4-oxopiperidine-1-oxide,2,2,6,6-tetramethylpiperazine-N-oxide, and chloranil.
 20. A processaccording to claim 1 wherein the polymerization inhibitor is in theconcentration range of 0.001 to 2% w/w of the total mixture of monomersand charger transfer agent.
 21. A process according to claim 20 whereinthe concentration range is 0.01 to 0.5% w/w.
 22. A process according toclaim 1 wherein the chain transfer agent is a solvent selected from thegroup consisting of chloroform, carbon tetrachloride, dimethylformamideand mixtures thereof.
 23. A process according to claim 1 wherein theconcentration of the monomers in the chain transfer solvent is in therange of 10 to 60% w/w.
 24. A process according to claim 23 wherein thesaid range is 30-50% w/w.
 25. A cation exchange resin made by theprocess of claim
 1. 26. A perm-selective membrane made from the resin ofclaim 26.